Propylene polymer composition and oriented film prepared thereby

ABSTRACT

The present invention provides a propylene polymer composition for producing a biaxially oriented film, comprising propylene random copolymer and propylene homopolymer; wherein the propylene random copolymer is a copolymer of propylene and ethylene, optionally comprising one or more alpha-olefins of C 4 -C 10 ; the propylene polymer composition has an overall isotacticity index, as determined by nuclear magnetic resonance method, of greater than or equal to 96.5%, and an ethylene content of greater than 0.3 wt %; and wherein, the melt flow rate of the propylene random copolymer is lower than that of the propylene homopolymer. The biaxially oriented polypropylene film produced by the propylene polymer composition of the present invention exhibits excellent physical properties, and has, in the absence of any stiffening agent, relatively higher modulus and stiffness. Moreover, the process for producing the biaxially oriented polypropylene film is featured with a good film-forming stability and a high film-forming stretching speed.

CROSS-REFERENCE OF THE RELATED APPLICATIONS

This application is based upon and claims priority of Chinese PatentApplication No. 200510004901.9 filed on Jan. 28, 2005, the contentsbeing incorporated herein for reference in their entirety for allpurposes.

TECHNICAL FIELD

The present invention relates to a propylene polymer composition, inparticular, to a propylene polymer composition for producing a biaxiallyoriented film and a method for preparing the same, and to a biaxiallyoriented polypropylene film (BOPP) prepared by the composition.

BACKGROUND ART

Owing to its excellent optical properties, mechanical properties andpackaging adaptability, polypropylene film has been widely applied tothe field of packaging material. In particular, biaxially orientedpolypropylene film (BOPP) has been widely applied to package variousproducts such as foods, medicines, commodities and cigarettes and alsoapplied as a substrate material for high-strength composite film, due toits features of softness, good transparency, non-toxicity, good waterrepellent property and high mechanical strength.

Biaxially oriented polypropylene film (BOPP) is generally produced withpolypropylene resin as raw material by the steps of: extruding the resinto obtain a film, and then subjecting the film to biaxial stretching,heat treatment and cooling treatment. To meet the requirement ofhigh-speed film production in a large scale, the polypropylene resin rawmaterial should have both good tensile strength and excellenthigh-temperature stretching property. As far as polypropylene isconcerned, it is generally deemed that a lower isotacticity index and alower melting temperature of the polymer are favorable to the processingand forming of the BOPP film, but the mechanical properties such astensile strength and stiffness of the film as obtained will be reduced.Thus, a method of adding C₅-C₉ petroleum resin to isotacticpolypropylene material was developed in order to increase modulus of thematerial, thereby improving the stiffness of the finally obtainedpolypropylene film. For instance, Chinese Patent CN1102419A disclosed apropylene polymer composition comprising C₅ hydrocarbons free of polargroup, the use of which can improve the steam barrier property,transparency and Young's modulus of the polypropylene oriented filmobtained thereby. However, this method not only needs extra componentsthereby increasing the production cost, but also does not have asatisfactory result.

In the prior art, the methods for preparing BOPP films with propylenerandom copolymers, in particular, compositions comprising propylenerandom copolymers, as raw materials are also disclosed, wherein therandom copolymers are generally copolymers of propylene andalpha-olefins such as ethylene. For instance, as disclosed in ChinesePatent CN1404500A, the amount of ethylene in propylene copolymer or inpropylene polymer composition should be greater than 0.8 wt %, generallyin the range of 0.8-1.5 wt %. Moreover, the MFR value of the firstpolypropylene (generally a homopolymer) is lower than that of the secondpropylene random copolymer, i.e., the molecular weight of the firstpolypropylene (generally a homopolymer) is higher than that of thesecond propylene random copolymer, and the difference in the MFR valuesbeing preferably greater than 5 g/10 min. Such control in ethylenecontent and melt flow rate in the propylene polymer composition might beapplicable to the special polymerization reactor and specialpolymerization method, wherein two interconnected gaseous polymerizationreaction zones are used, as adopted in the embodiment of that patentapplication. However, as to a common reactor or polymerization method,such control cannot balance the requirement of producing BOPP film andthe achievement of polypropylene resin having satisfactory inherentproperties, which mainly concern high soluble content of thepolypropylene resin and reduced transparency and stiffness of the film.

In view of the situations in the prior art, it is desired to develop apropylene polymer composition for producing a biaxially oriented film,which can better balance the high-temperature stretching properties andphysical properties of the film, thereby obtaining a BOPP film havingexcellent comprehensive properties.

The present inventors discovered by laborious experiments that byincreasing the isotacticity of the polymer having a relatively higherMFR value (i.e., a relatively lower molecular weight) in the propylenepolymer composition to keep the overall isotacticity of the propylenepolymer composition at a relatively higher level, a polypropyleneoriented film having high modulus, with its other properties such astransparency and mechanical properties being somewhat improved, can beobtained, without the addition of any stiffening agent (such as C₅-C₉petroleum resin)

CONTENTS OF THE INVENTION

The present invention provides a propylene polymer composition forproducing a biaxially oriented film, comprising propylene randomcopolymer and propylene homopolymer; wherein the propylene randomcopolymer is a copolymer of propylene and ethylene, optionallycomprising one or more alpha-olefins of C₄-C₁₀. The propylene polymercomposition has an overall isotacticity index, as determined by nuclearmagnetic resonance (NMR) method, of greater than or equal to 96.5%, andan ethylene content of greater than 0.3 wt %.

To obtain a BOPP film having high stiffness (including high transverseYoung modulus and longitudinal Young modulus), it is very important toprovide a propylene homopolymer having a relatively higher isotacticityindex. In principle, the higher, the better. In comparison with theprior art, the present invention differs in that: under the condition ofmaintaining a suitable range of ethylene content in the composition, theoverall isotacticity index of the composition is controlled at arelatively higher level (being completely higher than that in the priorart) by increasing the isotacticity index of propylene homopolymer,thereby achieving the object of increasing the stiffness of the film.

In the present invention, the overall isotacticity index of the finalpropylene polymer composition is determined by NMR method, wherein theethylene segments are regarded as defects. The formula for calculationis as follows:

$T_{overall} = \frac{\left\lbrack {m\; m} \right\rbrack + {\frac{1}{2}\left\lbrack {{{mr}\mspace{11mu}\&}\mspace{11mu}{PPE}} \right\rbrack}}{\left\lbrack {m\; m} \right\rbrack + \left\lbrack {{{mr}\mspace{11mu}\&}\mspace{11mu}{PPE}} \right\rbrack + \left\lbrack {{{rr}\mspace{11mu}\&}\mspace{11mu}{EPE}} \right\rbrack}$wherein, T_(overall) represents the overall isotacticity index of thepropylene polymer composition, [mm], [mr], [rr], [PPE] and [EPE] aremolar contents of the corresponding triads (characterized by thecorresponding peak areas after normalization).

A BOPP film having excellent properties can be obtained when the overallisotacticity index is greater than or equal to 96.5%, preferably greaterthan or equal to 97%, more preferably greater than or equal to 97.5%. Incontrast, the propylene copolymer resin useful for producing BOPP film,as disclosed in the prior art, generally has an overall isotacticityindex of less than 96%.

The crystallization distribution curve of the propylene polymercomposition described herein is determined by CRYSTAF method (describedin detail hereinafter) to obtain a Dispersion index R, whereinR=(T_(w)/T_(n)−1)×100, wherein T_(w) and T_(n) respectively representweight-average and number-average crystallization temperaturescalculated by CRYSTAF method. In the present invention, R is preferablyless than or equal to 2.5, more preferably less than or equal to 2.0. Incontrast, the R values of products known in the field are generallygreater than 2.5.

Other properties of the present propylene polymer composition forproducing the BOPP film are preferably as follows.

In the propylene polymer composition, the content of fraction soluble inxylene at room temperature (about 25° C.) is preferably less than 3 wt%, more preferably lower than 2.6 wt %, in particular preferably lowerthan or equal to 2.0 wt %. In general, the higher the content offraction soluble in xylene is, the lower rigidity the film will have,and which higher content possibly causes that the film cannot contactdirectly with food, medicine and the like as a packaging material, orthe article to be packaged may be polluted.

In order to obtain a BOPP film having excellent comprehensive propertiesand good processability, it is also very important to control themolecular weight and molecular weight distribution of the propylenepolymer composition. Herein, it is preferred that, in the propylenepolymer composition, the MFR value of the propylene random copolymer iscontrolled to be lower than that of the propylene homopolymer, i.e., themolecular weight of the propylene random copolymer is greater than thatof the propylene homopolymer. The MFR value is determined under a loadof 2.16 kg at 230° C. according to the method of ISO1133. Preferably,the propylene random copolymer has a MFR value of 0.05-0.5 g/10 min, andthe finally obtained propylene polymer composition has a MFR value of1-8 g/10 min. More preferably, the propylene random copolymer has a MFRvalue of 0.1-0.3 g/10 min, and the finally obtained propylene polymercomposition has a MFR value of 2-4 g/10 min. The propylene polymercomposition has a molecular weight distribution index (weight-averagemolecular weight/numeric-average molecular weight) of 4-10, preferably5-7.

For satisfying the above requirements about molecular weight andmolecular weight distribution, the mass ratio of propylene randomcopolymer and propylene homopolymer in the propylene polymer compositionis generally 30:70 to 70:30, preferably 65:35 to 35:65, more preferably55:45 to 45:55.

In one specific embodiment herein, it is usual to control the comomonerethylene content of the propylene random copolymer in the propylenepolymer composition, thereby making the ethylene content in the finallyobtained propylene polymer composition to be greater than 0.3 wt %,preferably 0.3-0.8 wt %. Moreover, the propylene random copolymerfurther optionally comprises one or more alpha-olefins of C₄-C₁₀, saidalpha-olefins of C₄-C₁₀ specifically including 1-butene, 1-pentene,1-hexene, 4-methyl-1-pentene and 1-octene, preferably 1-butene.

In one preferred embodiment described herein, the propylene polymercomposition is characterized by having:

(1) an overall isotacticity index, as determined by NMR method, ofgreater than or equal to 97%;

(2) an ethylene content of 0.3-0.8 wt %;

(3) a Dispersion index R of less than or equal to 2.0 according to thecrystallization distribution curve as determined by a CRYSTAF, model200;

(4) a content of fraction soluble in xylene at room temperature (about25° C.) less than 2.6 wt %; and

(5) a MFR value of 1-3 g/10 min, wherein the propylene random copolymerhaving a MFR value of 0.1-0.3 g/10 min.

The propylene polymer composition for producing biaxially oriented filmin the present invention may be prepared by one or more polymerizationsteps. Preferably, it is prepared, in the presence of a Ziegler-Nattacatalyst having high activity and high stereo-selectivity, by a methodcomprising two-step polymerization reactions, wherein one of the stepscomprises copolymerizing propylene with ethylene and optionally one ormore alpha-olefins of C₄-C₁₀ to obtain a propylene random copolymer, andthe other step comprises homopolymerizing propylene to obtain apropylene homopolymer. The two-step polymerization reactions may bestepwise carried out in different or same reaction zone(s). However, thetwo-step polymerization reactions are in an arbitrary order.

The polymerization reactions as above described may be carried out inpropylene liquid phase, or gas phase, or by using a liquid-gas combinedprocess. In liquid phase polymerization, the polymerization temperatureis 0-150° C., preferably 40-100° C.; and the polymerization pressure ishigher than the saturated vapor pressure of propylene under thepolymerization temperature. In gas phase polymerization, thepolymerization temperature is 0-150° C., preferably 40-100° C.; and thepolymerization pressure is normal pressure or higher, preferably 1.0-3.0MPa (gauge pressure, the same below). The polymerization may be carriedout continuously or intermittently. The continuous polymerization may becarried out by using two or more loop reactors_in series, or two or morekettle-type reactors in series, or two or more gas phase reactors inseries; or the combinations of loop reactor, kettle-type reactor and gasphase reactor. As to continuous liquid phase polymerization, thecatalyst generally needs to undergo continuous or intermittentprepolymerization. By prepolymerizing with propylene, the catalyst caneffectively control the particle shape of polymer during reaction, andreduce the rupture of polymer particles; and also the polymerizationactivity of the catalyst can be brought into full effect. Theprepolymerization reaction is generally carried out under relativelymilder conditions, preferably the polymerization temperature is lowerthan 30° C., and the prepolymerization multiple is controlled within3-1,000 times. In continuous gas phase polymerization, the catalyst maybe either prepolymerized or not.

Whether in the homopolymerization or coplymerization of propylene, amolecular weight regulator is used for regulating the molecular weightof polymer, so that the MFR of propylene random copolymer is lower thanthat of propylene homopolymer. The molecular weight regulator ispreferably hydrogen.

The polymerization reactions as above described are preferably carriedout in the presence of a Ziegler-Natta catalyst having highstereo-selectivity. The “Ziegler-Natta catalyst having highstereo-selectivity” used herein refers to the catalyst capable ofpreparing a propylene homopolymer having isotacticity index of greaterthan 97%, preferably greater than 98%, more preferably greater than 99%.This type of catalyst generally comprises active solid catalystcomponent, preferably a Ti-containing solid catalyst as an activecomponent and an organic aluminum compound as a co-catalyst component,to which an external electron donor component may be optionally added.

The concrete examples for this type of catalyst are disclosed in ChinesePatents: CN85100997A, CN1258680A, CN1258683A, CN1258684A, CN1091748A,CN1330086A, CN1298887A, CN1298888A and CN1436796A. These catalysts maybe directly used or prepolymerized prior to use. The catalysts asdisclosed in Chinese Patents: CN1330086A, CN85100997 and CN1258683A areparticularly favorable for use in the present invention.

The Ti-containing active solid catalyst component may be preparedaccording to various methods.

Usually, the catalyst active component is prepared by loading a Ticompound and an internal electron donor compound on a MgCl₂.nROH adduct,wherein the MgCl₂.nROH adduct is an adduct of MgCl₂ and alcohol,preferably in the form of spherical particles; and wherein n is usually2.0 to 3.5, R is an alkyl group having 1 to 4 carbon atoms, and thealcohol includes ethanol, propanol, isopropanol, butanol, isobutanol,isooctanol and etc. For the concrete preparation steps, one may refer toChinese Patents CN1036011C and CN1330086A.

Additionally, the Ti-containing solid catalyst component may also beprepared by referring to the following method as disclosed in ChinesePatents CN85100997 and CN1258683A:

Firstly, magnesium halide is dissolved with a solvent system consistingof an organic epoxide, an organic phosphorus compound and an inertdiluent to obtain a homogenous solution; the solution is mixed with a Ticompound, and then a solid is precipitated in the presence of anauxiliary precipitation; the solid is treated with an internal electrondonor compound thereby loading it on the solid, if necessary, the solidis further treated with titanium tetrahalide and an inert diluent,thereby obtaining the Ti-containing solid catalyst component; whereinthe auxiliary precipitation is one selected from organic anhydride,organic acid, ether and ketone. Wherein, the amounts of variouscomponents are, based on per mole of magnesium halide, as follows: theorganic epoxide 0.2-10 mol, the organic phosphorus compound 0.1-3 mol,the auxiliary precipitation 0.03-1.0 mol, the titanium compound 0.5-150mol, and the internal electron donor compound 0.01-5 mol, preferably0.05-1 mol.

The internal electron donor compound in the catalyst component isusually selected from aliphatic dicarboxylic acid esters or aromaticdicarboxylic acid esters, preferably, dialkyl phthalates, for example,e.g., diethyl phthalate, diisobutyl phthalate, di-n-butyl phthalate,diisooctyl phthalate, di-n-octyl phthalate, and etc.

The internal electron donor compound can also be selected from adihydric alcohol ester compound shown as in formula (I):

wherein, R₁-R₆ and R¹-R^(2n) radicals, which are same or different,represent hydrogen, halogen, or substituted or unsubstituted linear orbranched C₁-C₂₀ alkyl group, C₃-C₂₀ cycloalkyl group, C₆-C₂₀ aryl group,C₇-C₂₀ alkaryl, C₇-C₂₀ aralkyl, C₂-C₁₀ alkenyl, C₁₀-C₂₀ fused ring arylor C₂-C₁₀ ester group; but with the proviso that R₁ and R₂ are nothydrogen; R₃-R₆ and R¹-R^(2n) radicals optionally contain one or moreheteroatoms to substitute carbon or hydrogen atom(s) or both of them;said heteroatom is selected from the group consisting of nitrogen,oxygen, sulfur, silicon, phosphorus or halo atom; optionally, one ormore R₃-R₆ and R¹-R^(2n) radicals are linked together to form a ring; nis an integer of 0 to 10.

This type of dihydric alcohol ester compound is disclosed in ChinesePatents CN1436766A, CN1436796A and CN1453298A.

The organic aluminum compound as a co-catalyst component preferablyincludes alkyl aluminum compound, more preferably trialkylaluminum suchas triethylaluminum, triisobutylaluminum and tri-b-butyl aluminum. Inthe catalyst, the molar ratio of Ti/Al is 1:25-100.

In order to increase the overall isotacticity index of the final polymercomposition, in particular the isotacticity index of propylenehomopolymer in the composition, it is usually desired to introduce anexternal electron donor compound to the catalyst. The external electrondonor compound is preferably an organic silicon compound having aformula R_(n)Si(OR′)_(4−n), wherein 0<n≦3; R and R′, which are same ordifferent, represent alkyl group, cycloalkyl group, aryl group,haloalkyl group and etc.; R may also be halo or hydrogen atom.Specifically, the external electron donor compound includestrimethylmethoxysilane, trimethylethoxysilane, trimethylphenoxysilane,dimethyldimethoxysilane, dimethyldiethoxysilane, methyltert-butyldimethoxysilane, methylisopropyldimethoxysilane,diphenoxydimethoxysilane, diphenyldiethyoxysilane,phenyltrimethoxysilane, phenyltriethoxysilane, vinyltrimethoxysilane,cyclohexylmethyldimethoxysilane, dicyclopentyldimethoxysilane,diisopropyldimethoxysilane, diisobutyldimethoxysilane,2-ethylpiperidyl-2-tert-butyldimethoxysilane,(1,1,1-trifluoro-2-propyl)-2-ethylpiperidyldimethoxysilane and(1,1,1-trifluoro-2-propyl)-methyldimethoxysilane and etc., During thepolymerization reactions, the isotacticity index of polypropylene can beeffectively controlled through regulating the ratio between the organicaluminum compound as a co-catalyst component and the organic siliconcompound as an external electron donor component in the catalystdescribed herein. In order to obtain a propylene polymer compositionhaving the desired properties, the ratio of said two components, basedon the molar ratio of Al/Si, is preferably 4-30, more preferably 6-20.

In one preferred embodiment, the method for preparing the propylenepolymer composition described herein comprises subjecting propylene tocontinuous or intermittent random copolymerization andhomopolymerization reactions in two or more reaction zones in series;wherein, a molecular weight regulator such as hydrogen is added indifferent amounts to different reaction zones, so that the MFR ofpropylene copolymer is lower than that of propylene homopolymer; andwherein, the yield ratio of the copolymerization and homopolymerizationof propylene is 30:70 to 70:30, the polymerization temperature iscontrolled in the range of 60-80° C., and the polymerization reactionsare carried out in liquid phase.

Wherein, a specific embodiment includes subjecting propylene tocontinuous random copolymerization and homopolymerization reactions intwo loop reactors in series. Wherein, in the presence of a Ziegler-Nattacatalyst having high stereo-selectivity, the random copolymerizationreaction of propylene and ethylene is carried out in the first loopreactor for a certain time, thereafter the reactants are transferred tothe second loop reactor for carrying out the homopolymerization reactionof propylene; and wherein, hydrogen is added in different amounts to thefirst and second loop reactors, the hydrogen amount in the first loopreactor being lower than that in the second loop reactor, so that theMFR value of the propylene copolymer in the first stage is lower thanthat of the propylene homopolymer in the second stage. The yield ratioin the first and second loop reactors is about 65:35 to 55:45, thepolymerization temperature in the two loop reactors is controlled in therange of 60-80° C., and the polymerization reactions are liquid phasebulk reactions.

Another specific embodiment comprises subjecting propylene to continuousrandom copolymerization and homopolymerization reactions in twokettle-type reactors in series. Wherein, in the presence of the aboveZiegler-Natta catalyst of high efficiency as result of theprepolymerization, the random copolymerization reaction of propylene andethylene is carried out in the first kettle-type reactor for a certaintime, thereafter the reactants are transferred to the second kettle-typereactor for carrying out the homopolymerization reaction of propylene;and wherein, hydrogen is added in different amounts to the first andsecond kettle-type reactors, with the hydrogen amount in the firstkettle-type reactor being lower than that in the second kettle-typereactor. Alternatively, the homopolymerization reaction of propylene maybe firstly carried out prior to the copolymerization reaction ofpropylene and ethylene. No matter which of the steps is firstly carriedout, the MFR value of the propylene copolymer is lower than that of thepropylene homopolymer. The yield ratio of the copolymerization andhomopolymerization of propylene is about 65:35 to 55:45, thepolymerization temperature in the two kettle-type reactors is controlledin the range of 60-80° C., and the polymerization reactions are liquidphase bulk reactions.

The propylene polymer composition described herein can also be preparedby, apart from the above multi-step polymerization reactions, mechanicalblending in melt state by using a routine mixing device such as screwextruder.

The propylene polymer composition described herein can be used forproducing single- or multi-layer, uni- or bi-axially oriented film. Themulti-layer oriented film has at least one layer containing thepropylene polymer composition described herein.

The followings are directed in detail to BOPP film produced by thepropylene polymer composition described herein. Prior to producing theBOPP film, the propylene polymer composition is generally pelletized byextrusion, during which various additives conventionally used in thefield may be added, such as antioxidants, halogen-absord agents, lightstabilizers, heat stabilizers, colorants and fillers. The antioxidantsinclude phenols, phosphites and etc.; and the halogen-absord agentsinclude aliphatic metal salts.

The BOPP film described herein can be produced by various well-knownmethods, such as flat-film method and tubular-film method. The flat-filmmethod is often used. The method comprises the steps of: mixing rawmaterials, extruding, casting, longitudinally stretching, transverselystretching, edge cutting, treating with corona, coiling up, ageing,cutting, and packaging.

The BOPP film produced by the propylene polymer composition describedherein exhibits excellent physical properties, which has, in the absenceof any stiffening agent, relatively higher modulus and stiffness, withthe transverse Young modulus being ≧5,000 MPa, even ≧5,300 Mpa, and thelongitudinal Young modulus being ≧2,400 Mpa, even ≧2,600 Mpa. The filmhas a haze of no more than 0.5%. Owing to the special structure of thepropylene polymer composition described herein, the process forproducing the BOPP film is featured with a good film-forming stabilityand a film-forming stretching speed of greater than 380 m/min.

All the publications mentioned are incorporated herein for reference intheir entirety for all purposes.

Unless identified otherwise, the percentages, ratios and amounts usedherein are all on the basis of weight.

MODE OF CARRYING OUT THE INVENTION

The following examples are for illustrative purposes only and are not tobe construed as limiting this invention in any manner.

The methods for determining relevant data of polymers and films in thepresent invention and its examples are as follows.

1. The overall isotacticity index and comonomer content of the propylenepolymer composition are determined by the following method.

The overall isotacticity index T_(overall) and comonomer ethylene (E)content of the propylene polymer composition described herein aredetermined by using AVANCE 400-nuclear magnetic resonance (NMR)spectrometer by Bruker company in Germany. The sample is dissolved at140° C. using deuterated o-dichlorobenzene as solvent, and determined at125° C. using a probe of 10 mm with a delay time (D1) of 10 seconds, asampling time (AT) of 5 seconds, and a number of scanning of over 5,000times. The steps of test operation, identification of spectral peaks andmethod of data processing are performed according to the test standardof NMR, and one can refer to the following document for more details:(1) James C. Randall, A Review of High Resolution Liquid ¹³CarbonNuclear Magnetic Resonance Characterization of Ethylene-Based Polymers,JMS-REV. Macromol. Chem. Phys., C29 (2&3), 201-317 (1989). (2) VincenzoBusico, Roberta Cipulo, Guglielmo Monaco, and Michele Vacatello, FullAssignment of the ¹³Carbon NMR Spectra of Regioregular Polypropylenes:Methyl and Methylene Region, Macromolecules, 30, 6251-6263 (1997).

The formula for calculation of overall isotacticity index is as follows:

$T_{overall} = \frac{\left\lbrack {m\; m} \right\rbrack + {\frac{1}{2}\left\lbrack {{{mr}\mspace{11mu}\&}\mspace{11mu}{PPE}} \right\rbrack}}{\left\lbrack {m\; m} \right\rbrack + \left\lbrack {{{mr}\mspace{11mu}\&}\mspace{11mu}{PPE}} \right\rbrack + \left\lbrack {{{rr}\mspace{11mu}\&}\mspace{11mu}{EPE}} \right\rbrack}$

2. The Determination of Dispersion index (R)—CRYSTAF Method

The crystallization distribution curve is determined by using CRYSTAF,model 200 manufactured by Polymer Characterization S. A. Co., Spain,thereby obtaining the Dispersion index (R) by using a data processingmethod “others” therein. Wherein, 1,2,4-trichlorobenzene is used assolvent (to which 2,6-dibutyl-p-methylphenol as antioxidant is added ina concentration of 0.3 g/l), the sample amount is 30 mg, the temperatureof dissolving sample is set at 160° C., the rate of lowering temperatureis 0.2° C./min, and other operations are performed according to thespecification of the analyzer.

3. The melt flow rate (MFR) is determined under a load of 2.16 kg at230° C. according to the method described in ISO1133.

4. The molecular weight distribution is determined by gel permeationchromatography (GPC), and generally calibrated by a narrow-distributedpolystyrene standard sample.

5. The xylene-soluble is determined according to the method described inASTM D5492-98.

6. The melting temperature and crystallization temperature aredetermined according to the method described in ASTM D3418-03.

7. The resin tensile strength is determined according to the methoddescribed in ASTM D638-00.

8. The resin flexural modulus is determined according to the methoddescribed in ASTM D790-97.

9. The Izod impact strength is determined according to the methoddescribed in ASTM D256-00.

10. The Charpy impact strength is determined according to the methoddescribed in GB/T 1043.

11. The haze of film is determined according to the method described inASTM D1003-00.

12. The tensile strength and Young modulus of film are determinedaccording to the method described in GB/T 13022-1991.

13. The heat shrinkage of film is determined according to the methoddescribed in ASTM D1204-02.

Unless identified otherwise, all of the above determinations areconducted under atmospheric environment.

EXAMPLE 1

1. Preparation of a Catalyst Active Component

The catalyst active component was prepared according to the methoddescribed in Example 1 of Chinese Patent CN1330086A, comprising Ti 2.4wt %, Mg 18.0 wt %, and di-n-butyl phthalate 13 wt %.

2. Preparation of Propylene Polymer Composition

The polymerization reactions were continuously carried out in two loopreactors in series. Firstly, the above catalyst active component,triethylaluminum as co-catalyst component andcyclohexylmethyldimethoxysilane as external electron donor in a massratio of 1:10:2 were continuously added to a prepolymerization reactor,to which adequate liquid propylene was fed for prepolymerization at 15°C. for 6 min. The catalyst as prepolymerized was continuously added tothe first loop reactor, to which propylene, ethylene and hydrogen in amass ratio of 310000:1100:3 were fed to carry out the randomcopolymerization reaction of propylene for 1 h. Thereafter, thereactants were transferred to the second loop reactor for carrying outthe homopolymerization reaction of propylene, to which fresh propyleneand hydrogen in a mass ratio of 1500:1 were supplemented. Thepolymerization temperature in the two loop reactors was 70° C., and theyield ratio in the first and second loop reactors was controlled at60:40. The properties of the polymer composition as obtained in the formof powder were shown in Table 1. Each of the above steps was carried outunder a pressure higher than the saturated vapor pressure of propyleneat 70° C., and two liquid phase loop reactors in series were usedtherein.

3. Production of BOPP Film

0.1 wt % Irgafos168 additive (antioxidant), 0.2 wt % Irganox1010additive (antioxidant) and 0.1 wt % calcium stearate (halogen-absordagent) were added to the polymer powder as above obtained, which wasthen pelletized with a 65 mm twin-screw extruder at 220° C.

The materials as pelletized were extruded with a sheet extruder of Φ70mm at 220° C. By using a flat-film continuous biaxial stretching device,the sheet as extruded was longitudinally oriented by 10 times at 140° C.and then transversely oriented by 8 times at 170° C., thereby obtaininga A-B-A type biaxially oriented film having a thickness of about 20 μm,with outside layers and core layer being the same polymer. Theproperties of the film were shown in Table 3.

EXAMPLE 2

1. Preparation of a catalyst active component: the same as thatdescribed in Example 1.

2. Preparation of propylene polymer composition

The reaction conditions were identical with those described in Example1, except for that dicyclopentyldimethoxysilane was used as an externalelectron donor component, and the yield ratio in the first and secondloop reactors was controlled at 65:35. The properties of the compositionas obtained were shown in Table 1.

3. Production of BOPP film: the same as that described in Example 1. Theproperties of the film as obtained were shown in Table 3.

EXAMPLE 3

1. Preparation of a Catalyst Active Component

The catalyst active component was prepared according to the methoddescribed in Example 1 of Chinese Patent CN85100997, comprising Ti 2.03wt %, Mg 17.8 wt %, and diisobutyl phthalate 12 wt %.

2. Preparation of propylene polymer composition: the same as thatdescribed in Example 1. The properties of the composition as obtainedwere shown in Table 1.

3. Production of BOPP film: the same as that described in Example 1. Theproperties of the film as obtained were shown in Table 3.

EXAMPLE 4

1. Preparation of a catalyst active component: the same as thatdescribed in Example 3.

2. Preparation of propylene polymer composition: the same as thatdescribed in Example 2. The properties of the composition as obtainedwere shown in Table 1.

3. Production of BOPP film: the same as that described in Example 1. Theproperties of the film as obtained were shown in Table 3.

COMPARATIVE EXAMPLE 1

The polymer used was a high-quality propylene homopolymer FS3011 (ChissoCo, Japan) for producing high-stiffness BOPP film, available in ourcountry's market. A polypropylene biaxially oriented film was producedwith a flat-film continuous biaxial stretching device according to themethod described in Example 1.

COMPARATIVE EXAMPLE 2

1. Preparation of a catalyst active component: the same as thatdescribed in Example 3.

2. Preparation of propylene polymer composition

The polymerization reactions were continuously carried out in two loopreactors in series. Firstly, the catalyst active component,triethylaluminum as a co-catalyst component andcyclohexylmethyldimethoxysilane (in a mass ratio of 1:10:2) werecontinuously added to a prepolymerization reactor, to which adequateliquid propylene was fed for prepolymerization at 15° C. for 6 min. Thecatalyst as prepolymerized was continuously added to the first loopreactor, to which propylene, ethylene and hydrogen in a mass ratio of31000:110:6 were fed to carry out the random copolymerization reactionof propylene for 1 h. Thereafter, the reactants were transferred to thesecond loop reactor for carrying out the homopolymerization reaction ofpropylene, to which fresh propylene and hydrogen in a mass ratio of15000:3 were supplemented. The polymerization temperature in the twoloop reactors was 70° C., and the yield ratio in the first and secondloop reactors was controlled at 60:40. The properties of the polymercomposition as obtained in the form of powder were shown in Table 1.

The hydrogen amounts in the two reactors were controlled so that the MFRvalue of the propylene copolymer is higher than that of the propylenehomopolymer.

3. Production of BOPP film: the same as that described in Example 1. Theproperties of the film as obtained were shown in Table 3.

TABLE 1 Properties of the polymer compositions obtained in Examples 1-4and Comparative Examples 1-2 Comparative Example 1 Comparative Example 1Example 2 Example 3 Example 4 (FS3011) Example 2 MFR (g/10 Min) of 1.81.6 1.5 1.1 3.0 1.9 composition MFR (g/10 Min) of 0.23 0.24 0.29 0.27 —2.0 Discharge from the first reactor Ethylene content of 0.43 0.45 0.380.39 0 0.43 the composition (wt %) Weight-average 4.61 5.07 5.02 4.863.56 4.34 molecular weight (M_(w)/10⁵) Molecular weight 7.2 7.6 5.9 6.75.3 4.8 distribution index (M_(w)/M_(n)) Overall isotacticity 96.7 97.296.7 98.3 94.3 95.5 index of the composition (%) Dispersion index R 2.21.8 1.9 1.7 3.2 3.5 Xylene-soluble 2.6 1.7 2.0 1.7 3.8 3.8 (wt %)Melting 161.4 160.4 159.0 161.5 158.5 160.0 temperature (° C.)Crystallization 113.5 111.1 111.0 113.0 106.8 109.3 temperature (° C.)Tensile strength 41.8 43.3 41.2 44.8 43.2 38.2 (Mpa) Flexural modulus1.73 1.72 1.67 1.84 1.63 1.56 (Gpa) Izod impact 29.7 23.4 28.8 25.9 21.322.4 strength (J/m) Charpy impact 4.1 4.3 4.1 4.3 3.7 3.8 strength(KJ/m²)

TABLE 2 Comparison between the present invention and some polypropyleneresins for producing BOPP general-purpose film F280 (Shanghai 4792E1S28C Petrochemical Example 1 Example 2 Example 3 Example 4 (Exxon)(Basell) Co.) Ethylene content of 0.43 0.45 0.38 0.39 0.41 0.32 0.24 thecomposition (wt %) Overall isotacticity 96.7 97.2 96.7 98.3 95.3 93.895.4 index of the composition (%) Dispersion index R 2.2 1.8 1.9 1.7 2.93.1 2.5 Xylene-soluble 2.6 1.7 2.0 1.7 3.7 4.5 3.0 (wt %)

TABLE 3 Properties of BOPP films obtained in Examples 1-4 andComparative Examples 1-2 Comparative Comparative Example 1 Example 2Example 3 Example 4 Example 1 Example 2 Thickness (μm) 20 20 20 20 20 20Young modulus 2940/ 2971/ 2986/ 2984/ 2324/ 1890/ (Mpa) 5280 5991 50195578 4087 3767 Longitudinal/ transverse Tensile strength 202/285 210/305203/291 206/313 197/296 182/236 (Mpa) Longitudinal/ Transverse Heatshrinkage 2.7/1.1 2.4/1.1 2.5/1.1 2.1/1 2.8/1.3 2.8/1.4 (%)Longitudinal/ Transverse Haze (%) 0.5 0.5 0.5 0.5 0.5 0.7 Thickness+1~+1.5 +0.5~−4 +2~−3 +2~−3 +11.5~+5 +6~+5 derivation (%) Average +1.2−2 −0.8 −0.8 +8 +5.5 thickness derivation (%)

As seen from the data of BOPP films shown in Table 3, in comparison withthe high-stiffness BOPP films produced in the prior art (cf. ComparativeExamples 1-2), the BOPP films produced by the polymer compositions ofthe present invention (cf. Examples 1-4) had transverse Young modulushigher by 20-40%, and longitudinal Young modulus higher by 20%, as wellas higher tensile strength (longitudinal/transverse) and lower thicknessderivation. Meanwhile, other properties of the BOPP films describedherein are comparative to those in the Comparative Examples.

1. A propylene polymer composition comprising propylene random copolymerand propylene homopolymer; wherein the propylene random copolymer is acopolymer of propylene and ethylene, optionally comprising one or morealpha-olefins of C₄-C₁₀; the propylene polymer composition has anoverall isotacticity index, as determined by nuclear magnetic resonancemethod, of greater than or equal to 96.5%, and an ethylene content of0.3 wt % to 0.8 wt %.
 2. The propylene polymer composition according toclaim 1, wherein the melt flow rate of the propylene random copolymer islower than that of the propylene homopolymer.
 3. The propylene polymercomposition according to claim 1, which has an overall isotacticityindex of greater than or equal to 97%.
 4. The propylene polymercomposition according to claim 1, which has an overall isotacticityindex of greater than or equal to 97.5%.
 5. The propylene polymercomposition according to claim 1, which has a Dispersion index R of lessthan or equal to 2.5 according to the crystallization distribution curveas determined by a CRYSTAF model
 200. 6. The propylene polymercomposition according to claim 1, which has a Dispersion index R of lessthan or equal to 2.0 according to the crystallization distribution curveas determined by a CRYSTAF model
 200. 7. The propylene polymercomposition according to claim 1, wherein the content of fractionsoluble in xylene at room temperature 25° C. is less than 3.0 wt %. 8.The propylene polymer composition according to claim 1, wherein thepropylene random copolymer has a melt flow rate of 0.05-0.5 g/10 min,and the propylene polymer composition has a melt flow rate of 1-8 g/10min, as determined under a load of 2.16 kg at 230° C. according to themethod of ISO1133.
 9. The propylene polymer composition according toclaim 1, wherein the propylene random copolymer has a melt flow rate of0.1-0.3 g/10 min, and the propylene polymer composition has a melt flowrate of 1-3 g/10 min.
 10. The propylene polymer composition according toclaim 1, which has a molecular weight distribution index M_(w)/M_(n) of4-10.
 11. The propylene polymer composition according to claim 1,wherein the mass ratio of the propylene random copolymer and thepropylene homopolymer is 30:70-70:30.
 12. The propylene polymercomposition according to claim 1, comprising propylene random copolymerand propylene homopolymer, wherein the propylene random copolymer is acopolymer of propylene and ethylene, and the propylene polymercomposition is characterized by having: (1) an overall isotacticityindex, as determined by nuclear magnetic resonance method, of greaterthan or equal to 97%; (2) an ethylene content of 0.3-0.8 wt %; and (3) aDispersion index R of less than or equal to 2.5 according to thecrystallization distribution curve as determined by a CRYSTAF model 200.13. The propylene polymer composition according to claim 12, wherein thecontent of fraction soluble in xylene at room temperature 25° C. is lessthan 2.6 wt %, and the propylene polymer composition has a melt flowrate of 1-3 g/10 min, wherein the propylene random copolymer has a meltflow rate of 0.1-0.3 g/10 min.
 14. A method for preparing the propylenepolymer composition according to claim 1, which comprises, in thepresence of a Ziegler-Natta catalyst having high stereo-selectivity,two-step polymerization reactions as follows: copolymerizing propylenewith ethylene and optionally one or more alpha-olefins of C₄-C₁₀ toobtain a propylene random copolymer; and homopolymerizing propylene toobtain a propylene homopolymer; wherein, the amounts of molecular weightregulator used in the two-step polymerization reactions are controlledso that the melt flow rate of the propylene random copolymer is lowerthan that of the propylene homopolymer; and the yield ratio of thecopolymerization and homopolymerization of propylene is 30:70 to 70:30.15. The method according to claim 14, wherein the two-steppolymerization reactions are continuously carried out in at least tworeaction zones in series.
 16. A method according to claim 14, whichcomprises subjecting propylene to continuous random copolymerization andhomopolymerization reactions in two loop reactors in series; wherein, inthe presence of a Ziegler-Natta catalyst having high stereo-selectivity,the random copolymerization reaction of propylene and ethylene iscarried out in the first loop reactor for a certain time, thereafter thereactants are transferred to the second loop reactor for carrying outthe homopolymerization reaction of propylene; and wherein, a molecularweight regulator is added in different amounts to the first and secondloop reactors, so that the MFR value of the propylene copolymer in thefirst stage is lower than that of the propylene homopolymer in thesecond stage; the yield ratio in the first and second loop reactors isabout 65:35 to 55:45, the polymerization temperature in the two loopreactors is controlled in the range of 60-80° C., and the polymerizationreactions are liquid phase bulk reactions.
 17. A method according toclaim 14, wherein the Ziegler-Natta catalyst having highstereo-selectivity comprises a Ti-containing active solid catalystcomponent, an organic aluminum compound as a co-catalyst component, andan organic silicon compound as an external electron donor component. 18.A method according to claim 17, wherein, in the Ziegler-Natta catalysthaving high stereo-selectivity, the ratio of the organic aluminumcompound as a co-catalyst component and the organic silicon compound asan external electron donor component, based on the molar ratio of Al/Siis 4-30.
 19. An oriented film produced by the propylene polymercomposition according to claim 1, which is single- or multi-layer, uni-or bi-axially oriented film, and the multi-layer oriented film has atleast one layer containing said propylene polymer composition.
 20. Thefilm according to claim 19, wherein said oriented film is biaxiallyoriented film, and the biaxially oriented film has, in the absence ofany stiffening agent, a transverse Young modulus of 5,000 MPa, and alongitudinal Young modulus of 2,400 MPa.
 21. The propylene polymercomposition according to claim 1, wherein the propylene polymercomposition is prepared by a method which comprises, in the presence ofa Ziegler-Natta catalyst having high stereo-selectivity, a two-steppolymerization reaction as follows: copolymerizing propylene withethylene and optionally one or more alpha-olefins of C₄-C₁₀ to obtain apropylene random copolymer; and homopolymerizing propylene to obtain apropylene homopolymer; wherein, the amounts of molecular weightregulator used in the two-step polymerization reactions are controlledso that the melt flow rate of the propylene random copolymer is lowerthan that of the propylene homopolymer; and the yield ratio of thecopolymerization and homopolymerization of propylene is 30:70 to 70:30.